What is the effect of O'Conner's use of the phrase "good country people" throughout the story?
As with much of O'Connor's work, the titles often contain irony, and this irony is repeated in the plot of the story. The "good country people" are not really good - Mrs. Freeman is a chatterbox that is always bragging about her two daughters. Mrs. Hopewell, Joy/Hulga's mother, is very insistent that the Freemans are not "white trash" - but "good country people" because certainly a prideful woman such as herself would never have hired white trash to work on her farm. Manley Pointer is an evil con artist who winds up seducing Joy/Hulga, and then stealing her pride and joy, her wooden leg, so he is not "good country people" either. None of the characters are good country people.
Joy/Hulga imagines herself above everyone else and ridicules the "good country people" around her because she is educated, and they are not, so she is not "good country people" either. She fancies herself above the "good country people" but she is not. The use of this phrase throughout the story not only has the effect of pointing out the fact that the "good country people" are really "bad country people" but it also emphasizes O'Connor's theme of grace, which is Biblically based. Grace is unmerited favor, given by God to sinners - people that do not deserve it. Even though the characters in this story are not "good", the Bible teaches that Jesus was sent to redeem sinners, not good people. O'Connor subscribed to the Biblical teaching that "For while we were still sinners, Christ died for our sins." This idea of grace is evident in everything she ever wrote, and in her letters, she constantly reminded people that this was why she wrote. This also explains why her characters are often so "grotesque" and undeserving of grace (like Joy/Hulga). They are the ones that need grace the most.